The advantages of using solitons (or, more precisely, shape maintaining pulses of electromagnetic radiation that can exist in single mode optical fiber) in optical fiber communication arrangements, has been discussed in many publications of late. Typical are U.S. Pat. No. 4,406,516 issued on Sept. 27, 1983 to A. Hasegawa, an article by A. Hasegawa et al., Proceedings of the IEEE, Volume 69(9), pages 1145-1150 (1981), and an article by applicant that appeared in Physics World, September, 1989, page 29 et seq., all of which are incorporated herein by reference.
It is well known that, in order for a pulse to remain a soliton, all its parts are required to maintain a common state of polarization, in spite of a constant evolution in that state of polarization caused by the fiber's birefringence. However, it was not known whether a series of solitons launched with a common polarization and having a common history would, as they traverse a transmission system, emerge with a common state of polarization, on a pulse to pulse basis. A pulse's history can be modified by two things: (1) changes in the birefringence of the fiber segments making up the system, and (2) the spontaneous emission noise (ASE) that is superimposed at random on the individual pulses. The former changes tend to be on a very slow time scale (typically minutes or longer), so that they are easily compensated by an automatic polarization controller device at the output of the system. Thus, only effects of the ASE have the potential to cause pulse-to-pulse changes in the polarization state of the soliton stream.